1. Field of the Invention
The present invention relates generally to warning light devices, and more particularly to a device for producing integrated directional light from an LED light source.
2. Description of the Related Art
Lights designed to serve illumination, warning or signaling functions must produce light of different intensity, duration and pattern. Within any broad category, such as warning lights, lights designed for a particular application, e.g., aircraft warning lights, may have a very different required intensity and light pattern than a warning light designed for, e.g., an emergency vehicle such as a police car or fire truck.
The prior art contains numerous examples of alternative light sources, reflectors and lenses arranged to produce particular intensities and distributions of light suited for a particular purpose. Of primary concern to designers of lights are efficiency and accuracy. By efficiency, it is meant that lighting designers are concerned with producing the maximum amount of light measured in candelas per unit of energy applied and transforming that light into a useful pattern with minimal losses. The light fixture must also distribute the available light as accurately as possible in the desired pattern. Lenses and other means used for bending and shaping light cause light losses due to differences between the refractive index of the lens material and the air surrounding the lens. Any light that is scattered, i.e., not accurately directed in the desired pattern, is effectively lost by being dispersed.
Until recently, light-emitting diodes (LEDs), while efficient producers of light in terms of candelas per watt of energy used, were extremely limited in the quality of light produced (candela vs. viewing angle), rendering them unsuitable for many applications. The viewing angle is the angle, measured with respect to the axis through the center of the lens of the LED, where the light intensity has fallen to fifty (50%) of the on-axis intensity. For example, a very bright LED, producing 3 to 5 candela may have a very narrow viewing angle of 8 to 15 degrees.
Recent advances in LED technology have resulted in LEDs having significantly improved light output. High-output (high flux) LEDs may now be a practical light source for use in signaling and warning illumination. Even though high-output LEDs have significantly greater luminous flux than previous LEDs, the total luminous flux is still relatively small, e.g., in the range of 5 to 20 candela, but will have a very wide viewing angle of 110 to 160 degrees. Thus, these newer LEDs produce a xe2x80x9chalf globexe2x80x9d of light in contrast to a directed xe2x80x9cspotxe2x80x9d of light with the older LEDs. Thus it is necessary to accumulate multiple LEDs in a compact array and externally focus their light output to produce a light source with luminous intensity sufficient for many applications.
LEDs are attractive to lighting designers because the light they produce is typically of a very narrow spectral wavelength, e.g., of a single pure color, such as red, blue, green, amber, etc. In the prior art, to achieve a colored light output, white light was produced and typically filtered through a colored lens or other colored material, such as a colored glass bulb to produce the desired light color. This causes a very large waste of light and the electrical energy used to produce the light, making such prior art devices very inefficient. LEDs are extremely efficient producers of colored light because the particular chemical compound used in the die of the LED, when excited by electrical current, produces a monochromatic band of energy within the visible light spectrum. For example, a red LED will generate a narrow wavelength of light in the visible red spectrum, e.g., 625 nm +/xe2x88x9220 nm. No external color filtering is needed, significantly improving the efficiency of the light source. Further, LEDs are directional light sources. The light produced from an LED is primarily directed along an optical axis through the center of the lens of the LED. However, and in particular with the more recent high-output LEDs, a significant portion of the light is also directed out the sides of the lens of the LED (the above mentioned xe2x80x9chalf globexe2x80x9d). If the limited light output of an LED is to result in a practical signaling or illuminating device, as much of the light produced by each LED must be captured and directed in the desired light pattern as possible.
U.S. Pat. No. 6,318,886, assigned to the assignee of the present invention, the entire contents of which are hereby incorporated by reference into this specification, describes a high-flux LED assembly in which an array of LEDs are provided with a reflector surrounding each LED. A conical reflecting surface collects and redirects light escaping from the LED at a large angle relative to the LED optical axis. The conical reflectors redirect such xe2x80x9cwide anglexe2x80x9d light out the face of the assembly, increasing the effective light contribution of each LED. The high-flux LED assembly also discloses connecting the conical reflectors with grooves to improve the wide-angle visibility desirable in a warning or signaling light application. By concentrating a number of high-output LEDs in a relatively small area and reflecting the light produced in a desired pattern, a very efficient and effective signaling and/or warning light is provided.
U.S. Pat. No. 6,318,886 mentioned above discloses an approach using conical reflectors. While the high-flux LED assembly described in the ""886 patent has proved successful for its desired application, further efficiencies are possible. The conical reflectors disclosed in the ""886 patent redirect light incident upon them out the face of the light assembly over a range of angles where the angle of the escaping light depends on the angular relationship between incident light and the reflecting surface. Such an arrangement, while desirably redirecting light out the front face of the assembly, undesirably does so over a range of angles. Some of the reflected light reinforces light output of the LED. Other light is reflected at random angles that fail to reinforce the light output of the LED and is effectively lost by being dispersed. The light pattern produced is essentially a series of bright points of light having somewhat improved wide-angle visibility due to the grooves connecting the conical reflectors.
It is known in the art to use parabolic reflectors to collimate the light output from prior art light sources such as halogen bulbs or xenon flash tubes. U.S. Pat. Nos. 4,792,717 and 4,886,329, both directed to a wide-angle warning light and both assigned to the assignee of the present invention, disclose the use of a parabolic reflector comprised of a linear parabolic section including parabolic dish ends. The reflector is configured with a reflector having a linear focal axis similar in configuration to the extended length of the xenon flash tube light source.
As exemplified by the ""886, ""717 and ""329 patents discussed above, reflectors for light assemblies are typically configured to complement the form of the light source, e.g., point light sources are provided with reflectors having axial symmetry and linear light sources are provided with reflectors having linear symmetry. The conventional approach generally involved matching the reflector to the light source to produce maximum light output from a light assembly.
Briefly stated, the present invention in a preferred form utilizes a linear array of equidistantly spaced point source LEDs arranged along the focal axis of a parabolic reflector having a linear parabolic section to produce a versatile light source whose output is primarily in the form of directional or collimated light. The linear parabolic reflector may comprise parabolic dish ends to form a light source whose output is in the form of a discrete bar of directional light. Alternatively, the linear parabolic reflector may be open at one or both ends to permit assembly into extended-length light sources. Parabolic dish ends may be omitted for the alternative purpose of improving wide-angle visibility if so desired.
A linear array of point source LEDs has a number of advantages over the prior art light sources, including efficiency, primarily collimated light output, pure color light output and durability. Additionally, a linear LED array in accordance with the present invention can be produced in any desired length.
In accordance with the present invention, a linear array of high-output LEDs is arranged on a printed circuit board (PC board). The PC board is then mounted such that each LED in the array is positioned along the linear focal axis of a reflector having a linear parabolic section. This may require a linear parabolic reflector with a slot-like opening adjacent the focal axis of the parabolic section. A reflector in accordance with the present invention is arranged to redirect xe2x80x9cwide anglexe2x80x9d light (light emitted at a large angle relative, to the optical axis of the LED) so that such light emerges from the reflector in a plane parallel to the optical axes of the LEDs. Even though the LEDs are distinct point light sources, the parabolic reflector combines the overlapping xe2x80x9cwide anglexe2x80x9d light of adjacent LEDs such that the light output from the assembly is substantially uniform, rather than closely spaced points of light, as would be expected.
For a variety of reasons, at least some of the light emitted at a small angle relative to the optical axis of an LED is not collimated. In accordance with another aspect of the invention, a collimating lens is arranged to redirect light output of the LED that would otherwise miss the parabolic reflector. Such a lens may be a simple convex flat lens or may be a Fresnel lens. In a manner similar to that of the lens on a lighthouse, light entering the collimating lens is refracted into a plane generally parallel to a plane including the optical axes of the LEDs. The collimating lens redirects light that would otherwise not contribute to the effective light output of the LED light assembly.
The combination of LED light sources, a parabolic reflector and a further collimating lens arranged adjacent the optical axes of the LEDs results in a light assembly from which substantially all of the light output is transmitted in the form of an organized wide angle beam. The wide angle beam can then be focused and/or spread to suit the desired application. A precisely configured beam such as that produced from the inventive arrangement results in a very precisely defined light pattern with little or no scattered light.
An object of the present invention is to provide a new and improved LED light assembly whose light output is primarily in the form of a wide angle beam.
Another object of the present invention is to provide a new and improved LED light assembly that efficiently integrates the light output of a plurality of point source LEDs into a substantially uniform light pattern.
A further object of the present invention is to provide a new and improved LED light assembly which efficiently uses a high-output LED to produce highly favorable illumination characteristics.